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In the scope of broadband radiators, the biconical antenna, or its monopole conical counterpart, is long known to be a proper choice. One common form of such radiator, the spherically capped conical antenna (SCCA), has closed-form solution to its input impedance, from which the broadband performance potential is easily verified. Nonetheless, from the design perspective, apart from a few clues inferred from existing solutions, little is found to accurately guide the choice of the main geometrical parameters of the antenna that will enable it to comply with a set of imposed bandwidth requirements. This paper proposes a simple 10-step sequence to derive conical or biconical antenna design charts. These charts provide straightforward information on the geometrical limits within which the required antenna impedance matching broadband performance is achieved. The method is assessed for the SCCA and the open conical antenna (OCA) using theoretical and simulated estimates of the input impedance. A discussion on the impact of the cap and the feed gap is included.

Biconical and conical antennas are among the most widely known radiators. They are natural choices for RF communication, broadcasting, or EMI testing, whenever omnidirectional radiation pattern and broadband performance are needed. The basic conical geometry (or biconical in its dipole equivalent) is typically assembled either by a wire grid or by a continuous metal surface [

The ideal biconical geometry is actually a frequency-independent antenna, though it is not a feasible one, since it extends to infinity in the axial direction. Its input impedance does not change with frequency. Realistic biconical antennas must be truncated, leading to a broadband rather than frequency-independent response [

Truncated versions of the biconical and conical antenna have been addressed as early as the 1940s by Schelkunoff [

A typical question that may arise for the designer of such antenna is how its geometric parameters should be chosen from start. Yet, how can one combine compactness and broadband performance up to the required levels considering such a structure? Though a few clues may be inferred from the mere observation of commercially available antennas, reliable open-source information to accurately aid the antenna engineer in this sense is not easily found.

In the present scope, this work discusses a simple method to derive bandwidth compliance charts for the design of conical and biconical antennas. These design charts set the limits that the main geometric parameters must fall within so that the antenna should be able to comply with a given imposed bandwidth constraint. The method is easily applied to any variant of the conical or biconical antenna, provided that a series of impedance estimates spanning a frequency band large enough and some different flare angle values is obtained. Such estimates may be derived analytically, for instance, when closed-form equations are available, as in the case of the spherically capped conical antenna (SCCA) addressed in [

Section

The reference antenna in the present work is the SCCA fed by a coaxial line depicted in Figure

Vertical plane cut of an SCCA fed by a coaxial feed line. The antenna is a body of revolution over the

It must be remarked that (

If the biconical equivalent of the SCCA is to be analyzed instead, the set of equations (

Figure ^{−4}. The broadband potential is clear as the antenna is large (high

SCCA input impedance versus

A proper assessment of how broadband the antenna may be depends not only on the antenna impedance itself but also on the reference impedance

Figure

As expected from the damped oscillatory behavior of the SCCA impedance, the most critical region for achieving wideband matching is at the lowest values of

To sum up, the antenna designer cannot rely exclusively on

As addressed in the previous section, the broadband antenna design point of view goes beyond what is inferred from the impedance response alone. The impedance dependence on the antenna main geometrical parameters must be previously mapped and translated to bandwidth performance that must comply with imposed project requirements, expressed in terms of a reference impedance and reflection coefficient thresholds.

In this sense, a general method to derive bandwidth compliance charts to aid the conical or biconical antenna design is proposed step by step, as follows. It is explained taking the SCCA theoretical model briefly reproduced in Section

(i) Calculate a reasonable number

(ii) From the design requirements, define the reference impedance (

(iii) Yet, from the design requirements, define a reference passband threshold, say −6 or −10 dB, and a maximum

(iv) Choose a criterion to define the lowest operation

Lowest operation

(v) Choose the bandwidth metric, which could be either the relative bandwidth

(vi) For each of

(vii) Plot

(viii) Plot

(ix) From the design requirements, define the minimum desired target for

(x) From the intermediary design charts

Design chart for the theoretical SCCA example with

The specific results of this example show that the SCCA is able to provide 50 Ω broadband impedance matching even with lengths smaller than

The availability of a closed-form solution for the impedance of the SCCA (or its biconical counterpart) surely eases the generation of design charts such as the proposed. Nonetheless, the rationale still applies to other variants of the conical antenna, such as the skeleton biconical antenna [

The OCA configuration, depicted in Figure

(a) Vertical plane cut of an OCA fed by a coaxial feed line and (b) a close-up of its junction. The antenna is a body of revolution over the

In order to apply the design chart procedure of Section

Both SCCA and OCA configurations were simulated in CST for

CST GUI views of the (a) OCA and (b) SCCA.

The CST setup for simulation was similar to the ones adopted in [

Figures

The corresponding reflection coefficients for the present example are shown in Figures

Design chart for the CST simulated SCCA example with

Design chart for the CST simulated OCA example with

The previous analysis pointed out the gap feed length as a relatively more impacting parameter than the spherical cap. Though the relevance of the feed gap to the conical antenna impedance and radiation performances should be expected, as addressed in [

In this sense, an extension to the feed gap analysis was carried out for the OCA configuration, calculating the impedance for two further gap lengths: 2 and 4 mm. Figures

The actual impact on the broadband performance is seen in the intermediary design charts in Figures

In this paper, a 10-step sequence was proposed to derive charts that directly relate impedance matching bandwidth compliance to the main geometrical parameters of conical or biconical antennas. The motivation came from the challenge faced by the antenna engineer when specific broadband impedance requirements are imposed. From the design perspective, the choice for the conical or biconical configuration leads to the subsequent questions on how long and how opened must the antenna be to meet the specifications. The proposed charts try to provide straightforward answers to those queries.

The method was assessed taking the SCCA and the OCA configurations, using theoretical and simulated estimates of the input impedance. A hypothetical set of requirements was imposed, with a reference impedance of 50 Ω, −6 and −10 dB

The authors declare that there is no conflict of interests regarding the publication of this paper.